1. Field of the Invention
The present invention relates to a honeycomb filter. More particularly, it relates to a honeycomb filter capable of suitably collecting particulate matters included in an exhaust gas discharged from an engine, especially an engine of a car.
2. Description of Related Art
A diesel engine is advantageous as an engine for a car which has a better heat efficiency than a gasoline engine and which matches a demand for a decrease of carbon dioxide emissions as a global warming countermeasure. On the other hand, the diesel engine discharges an exhaust gas including a large amount of particulate matters by diffusion combustion.
It has been pointed out that the particulate matters in the exhaust gas affect environments and human bodies, and measures to prevent the particulate matters in the exhaust gas from being discharged to the atmospheric air have been required. Heretofore, the amount of the particulate matters in the exhaust gas to be discharged from the diesel engine has been regulated in accordance with a mass ratio of the particulate matters in the exhaust gas. Moreover, in recent years, in addition to the above-mentioned regulation in accordance with the mass ratio of the particulate matters, a stricter exhaust gas regulation has been performed on the number of particulates in the exhaust gas.
An example of a method of decreasing a mass of the particulate matters or the number of the particulates in the exhaust gas discharged from the diesel engine is a method of improving the combustion in the diesel engine. However, in such a method of improving the combustion, there is a limit to the decrease of the mass of the particulate matters or the number of the particulates. Therefore, it is considered that a method of disposing a filter in an exhaust system of the diesel engine to collect the particulate matters in the exhaust gas by this filter is an effective method for decreasing the mass of the particulate matters or the number of the particulates in the exhaust gas.
An example of the filter to collect the particulate matters in the exhaust gas is a wall flow type filter including porous partition walls. In such a wall flow type filter, the exhaust gas permeates the porous partition walls, and the particulate matters in the exhaust gas are collected by the porous partition walls.
In the wall flow type filter, for the purpose of decreasing a pressure loss, it is necessary to lower a filtration speed. Therefore, in recent years, as the filter to collect the particulate matters in the exhaust gas, there has broadly been used a honeycomb filter using a honeycomb structure including a plurality of cells defined and formed by porous partition walls. In this honeycomb filter, among the cells formed in the honeycomb structure, open frontal areas of the predetermined cells in one end surface and open frontal areas of the remaining cells in the other end surface are plugged by plugged portions. The honeycomb filter has the honeycomb structure including the plurality of cells defined and formed by the partition walls, and hence a filtering area through which the exhaust gas passes can be increased.
When a particulate matter such as soot in the exhaust gas is collected by the honeycomb filter, the collected particulate matter is deposited on the partition walls. When the particulate matters are continuously collected by the honeycomb filter, the pressure loss of the honeycomb filter increases due to the particulate matters deposited on the partition walls. For decreasing the pressure loss of the honeycomb filter, it is necessary to forcibly burn and remove the particulate matters by applying a high temperature gas to the particulate matters deposited on the partition walls. The burning and removing of the particulate matters is often referred to as regeneration of filter.
At the regeneration of the filter, a heat stress is generated in the filter by heat of the burning of a particulate matter such as soot, and the generated heat stress causes damage to the filter which is being regenerated, sometimes. To prevent the filter from being damaged by the heat stress, there has been suggested a technology in which a plurality of honeycomb segments are prepared, and the plurality of honeycomb segments are bonded to manufacture a honeycomb filter (e.g., see Patent Document 1). In the honeycomb filter disclosed in Patent Document 1, the plurality of honeycomb segments are bonded by an easily deformable bonding material having a small elastic modulus. Hereinafter, the honeycomb filter manufactured by bonding the plurality of honeycomb segments will be referred to as “honeycomb filter having segment structure” sometimes.